Since titanium-based alloys have high reactivity at a high temperature, materials to be utilized as a die for molding or superplastic forming thereof are restricted extremely. Materials conventionally utilized for a die for casting include, (1) copper and mild steel, (2) graphite, (3) oxides such as zirconium, rare earth elements and calcium, and (4) metals with a high melting point such as niobium and molybdenum (Kei Sato and Takashi Yoneda, "Casting Technology of Titanium", JACT News, No. 459, published on Mar. 20, 1995, pp. 21-28). On the other hand, though superplastic forming is performed at a lower temperature than molding, calcium oxide not reacting with titanium-based alloys is also employed as a main component of a die (Makoto Kato and Hitoshi Murakami, "On the Dental Application of Titanium-based Alloy (Part 2) Ceramic Die for Superplastic Forming", Reports of the Government Industrial Research Institute, Nagoya, Vol. 39, Nos. 7-8, pp. 309-317, 1990). Since these dies are expensive or have a tendency to deteriorate owing to water existing in the atmosphere, the development of a die to be used more conveniently has been desired. In particular, if a die for superplastic forming with a high-workability not requiring the melting of titanium-based alloys is obtained, it can be possible to perform forming of titanium-based alloys conveniently.
Thus, dies conventionally used for forming of titanium-based alloys are not so sufficient. In particular, if a die for superplastic forming with a high-workability not requiring the melting of titanium-based alloys is obtained, the forming of titanium-based alloys can be performed easily. In view of the above prior arts, the present inventors have engaged in assiduous studies with a view to developing a die for superplastic forming capable of forming of titanium-based alloys easily, and as a result have succeeded in preparing a die with a high-workability comprising quartz, cristobalite and calcium silicate, which has led to the accomplishment of the present invention.